Organic chemistry B

Chapter 12Alkynes

By Prof. Dr.Adel M. Awadallah

Islamic University of Gaza

Alkenes and AlkynesHydrocarbons (contain only carbon and hydrogen)

a) Saturated: (Contain only single bonds)

Alkanes (CnH2N + 2 )

Cycloalkanes (CnH2N )

b) Unsaturated: contain

Alkenes: double bonds (,,,CnH2N)

Alkynes: triple bonds ((CnH2N - 2)

Aromatic: benzene like compounds

Facts about double and triple bonds

HH

bond angle 109.5 120 o 180 o

bond length 154 pm 134 pm 121 pm

rotation possible restricted restricted

geometry tetrahedral triagonal planer linear

Hypridization sp3 sp2 sp

Bond Length in Benzene 139 pm (plannar, sp2 hypridized)

• Bonding in acetylene (ethyne)

Nomenclature of alkenes and Alkynes1) The ending ene is used for alkenes and yne for alkynes

2) Select the longest chain that includes both carbons of the multiple bond

3) Number the chain from the end nearest to the multiple bond

4) Indicate the position of the multiple.

Examples

CH3

CH3

CH3

BrCH3 C

CHCH3

1

12 3

4

5

4-bromo-2-methyl-2-pentene

12

3456

4-methyl-1-hexyne

Assigning Priority

• Alkenes and alkynes are considered to have equal priority

• In a molecule with both a double and a triple bond, whichever is closer to the end of the chain determines the direction of numbering.

• In the case where each would have the same position number, the double bond takes the lower number.

• In the name, “ene” comes before “yne” because of alphabetization.

1234

567 1

23456

7

2-hepten-4-yne 4-hepten-2-yne

1234

567

2-hepten-5-yne not 5-hepten-2-yne

12

34

56 7

Preparation of Alkynes• 1) Generation of a triple bond• Note1) We need a very strong base for the second

dehydrohalogenation, since vinylic halides are very unreactive

• Note2): This is a method for preparation of vinilyc halides since substitution of alkenes is difficult

• 2) Increasing the size of a terminal alkyne

Reduction of alkynes With hydrogen in presence of ordinary nickel, platinum or palladium gives alkanesHydrogentaion of alkynes with Lindlar’s catalyst gives a cis alkene

•A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead. The lead additive serves to deactivate the palladium sites. A variety of "catalyst poisons" have been used including lead acetate and lead oxide. The palladium content of the catalyst is usually 5% by weight. The catalyst is used for the

hydrogenation of alkynes to alkenes.•As described by its inventor,[1][2] the catalyst is

prepared by reduction of palladium chloride in a slurry of calcium carbonate followed by adding lead acetate. By this approach, one obtains a catalyst with a large surface area. Further deactivation of the catalyst with quinoline enhances its selectivity, preventing formation of alkanes. An example of alkyne reduction is the reduction of phenylacetylene to styrene.[1]

•Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene.[3]

•Lindlar’s Catalyst